FIG. 1 shows a portion of a typical coal-burning utility electrical power generation plant 20. Coal is loaded into hopper 22 and gravity fed as shown at arrow 24 to inlet duct 26 of ball tube mill 28. The mill includes a rotary drum with a charge or plurality of balls therein which pulverize the coal as the drum rotates to produce pulverized coal. The pulverized coal is discharged as shown at arrow 34 to outlet duct 36 for introduction to the burner of boiler 38. Incoming combustion air is supplied as shown at arrow 40 from fan 42. The heat of combustion of the air and pulverized coal is supplied as shown at arrow 44 through a plurality of heat exchangers 46, 48, to generate steam to in turn drive electrical power generating turbines, as is standard. After giving up heat for the noted steam generation, exhaust including fly ash flows as shown at arrow 50 to exhaust outlet 52. An air preheater 54 is connected between combustion air inlet duct 56 and exhaust outlet duct 58 for transferring heat from the exhaust flow at 50 to the incoming combustion air flow at 40 to pre-heat the latter. The spent pulverized coal, after ignition and burning, fuses as hardened chunks called bottom ash clinkers, and are collected and ground at clinker grinder 60 for discharge and disposal. Of course, as will be understood by one of ordinary skill in the art, clinkers are also produced by other type coal-fired furnaces, beyond that depicted in FIG. 1.
A typical clinker grinder 60 includes two grinder roll assemblies that intermesh with knurls to grind the bottom ash clinkers. FIG. 2 depicts a conventional and well known grinder roll assembly 200. Included in assembly 200 is a cast Ni-hard grinding roll 205 that is mounted on a stainless steel shaft 210. As shown, the Ni-hard grinding roll 205 has multiple knurls, though for simplicity's sake only a single knurl is labeled 220, formed on the outer surface thereof. The Ni-hard grinding roll 205 cannot be directly welded to the stainless steel shaft 210 because of differences in material properties. In lieu of welding, pentagonal shaped insert 225 is used to mount the grinding roll 205 to the stainless steel shaft 210, as shown in FIG. 3. Insert 225 is hand fitted into the bore of the Ni-hard grinding roll 205. As shown in FIG. 4, the insert 225 is then welded to the stainless steel shaft 210.
A clinker grinder 60 is subject to extensive wear. In addition to wear to the knurls and other parts of the Ni-hard grinding roll 205, there is frequent failure of the weld between the insert 225 and the shaft 210. Also, it is known for insert 225 to fail and spin within the Ni-hard grinding roll 205. Because of the use of insert 225, both the Ni-hard grinding roll 205 and the stainless steel shaft 210 must be replaced together, even if only the Ni-hard grinding roll 205 is worn. That is, the welding of the insert 225 to the stainless steel shaft 210 causes stress concentrations to the shaft 210 that make it undesirable to reuse the stainless steel shaft 210.
The forced replacement of stainless steel shaft 210 is not economical, as each stainless steel shaft 210 costs approximately $1,200.00. Also, labor costs associated with machining the bore of the Ni-hard grinding roll 205 and the shaft 210 for fitting to the insert 225, and with the actual mounting of the Ni-hard grinding roll 205 on the shaft 210, are high. Furthermore, the mounting procedure itself is prone to crack the Ni-hard grinding roll 205, making it unusable.
Accordingly, a need exists for a grinder roll assembly which overcomes deficiencies of existing grinder roll assemblies. Thus, a need exists for a grinder roll assembly with an improved way of mounting a grinding roll to a shaft. A need also exists for a grinder roll assembly in which a welded insert is not required to mount the grinding roll to the shaft. A need also exists for a grinder roll assembly in which it is only necessary to replace worn parts. Also, a need exists for a grinder roll assembly which can be rebuilt using a previously used shaft, as well as a grinder roll assembly which is not subject to cracking during assembly. And, a need also exists for a grinder roll assembly which is economical to assemble.